This invention relates to an optical disc on which information is recorded, or from which the information recorded is reproduced, by radiation of a light beam from a laser, and more particularly to an optical disc suitable for recording information at a high density.
An optical disc of a so-called phase-changing type is known in the art. The optical disc of this type includes a transparent substrate on which at least a phase-changing material layer, a protective layer and a reflecting layer are formed. Further, the reflecting layer is covered with an protective over-coating layer, if necessary.
In the optical disc of such a type, the phase-changing material layer is initially maintained in a crystalline state. When a write beam from a laser is radiated on a predetermined spot-like portion of a surface of the optical disc, the portion of the phase-changing material layer is allowed to change from a crystal phase to a liquid phase. Thereafter, the liquid phase portion of the phase-changing material layer is rapidly cooled to form an amorphous phase portion in the form of pit. As a result, the amorphous phase portion of the phase-changing material layer formed by radiation of the write beam from the laser is caused to have a different reflection coefficient from the inherent crystal phase portion thereof. A read-out (reproduction) of the information recorded is performed by detecting a signal indicative of change (reduction) in the reflection coefficient of the amorphous phase portion.
Meanwhile, in the case of the optical disc of such a phase-changing type, the recording has been generally carried out by using a pit position-recording method.
In addition to this recording method, there has been also known a so-called pit length-recording method capable of recording an information at a high density. However, the pit length-recording method has such a problem that the recording accuracy and therefore the exact reproduction of data recorded is likely to be adversely affected due to possible fluctuation of the position of the amorphous pit formed on the surface of the optical disc. Consequently, in the case where the recording of information signal on the optical disc is carried out by using the pit length-recording method, it is desired that the layer structure on the substrate is so formed that the fluctuation of the pit position is unlikely to occur.
In the known optical disc, in order to prevent occurrence of the fluctuation of the pit position, the inherent crystal phase portion of the phase-changing material layer has a higher light absorptivity, i.e., a higher thermal absorptivity than that of the amorphous phase portion thereof which is formed by radiation of the light beam. However, in the optical disc having such a construction, the phase-changing material layer can be cooled only at a slow rate due to the high absorptivity of the crystal phase portion, so that there occurs a problem that the writing of the information signal must be performed at a line speed of 10 m/second or greater to avoid too much retention of heat in the crystal phase portion. The present invention has been made to overcome the above-mentioned problems encountered in the prior art.